What is the index in the select_linked_pick command?
For the operator bpy.ops.mesh.select_linked_pick(deselect=False, delimit={'SEAM'}, index=1729)  the index is taken from the closest element to the mouse cursor and it is calculated as follows :


For Vertices : index = vertex.index.
For Edges    : index = len(vertices)+ edge.index.
for Faces    :  index = len(vertices)+ len(edges) + face.index